We obtain your erroneous plot as the first component of a list, and the list of all the values passed to your ColorFunction: {{0., 0.2, 0.4, 0.6, 0.8, 1.}} as the second component. Effectively, what was done is to apply Rescale[] to the original list you plotted (i.e. Rescale[N@{2, 4, 6, 8, 10, 12}]). That's because ColorFunctionScaling is by default set to True:

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